Lithography has a variety of useful applications, including the manufacture of semiconductor devices, flat-panel displays, disk heads and the like. Designers and semiconductor device manufacturers constantly strive to develop smaller devices from, for example, semiconductor wafers, recognizing that circuits with smaller features generally produce greater speeds and increased yield. However, with smaller devices, it is becoming increasingly difficult to meet required critical dimension (CD) tolerances. Minor variations in various process parameters of the lithography exposure equipment (e.g. scanners/steppers), may cause the feature CDs to fall outside acceptable tolerance (e.g. +/−8%−10%).
The quality of the image pattern and feature CD may be affected by process variables, such as exposure dose and focus. Variations of dose can occur and may be caused by variations in the resist sensitivity, resist thickness, bake temperature and delay, anti-reflection coating and substrate films' thickness and optical constants. Variations of focus can occur and may be caused by, for example, variations in the substrate topography, substrate and mask/reticle chucking or flatness, errors of lithography system's auto-focus system, imperfections of focus servo control loop and interactions with product/layer stepping and focusing sequence, as well as focus drift. Variations in these process parameters can also result in errors of pattern placement manifested as feature-/layout-dependent layer-to-layer overlay (O/L) variations. As the feature CD gets smaller for smaller devices, the need for reduction of such errors increases. Imaged patterns require effective process monitoring to determine if the lithography process is within an acceptable tolerance range. Adjustments to dose and focus, for example, may need to be made to keep feature size and placement within acceptable tolerances.
CD and O/L are periodically checked to assure product quality, sometimes as infrequent as weekly, which leaves many semiconductor devices being processed to be at risk before errors are detected and corrections/adjustments are made. CD-related process monitoring may be done in a variety of ways, including, for example, using test substrates or test features, as well as patterns produced on production substrates. CD is typically measured using a critical dimension scanning electron microscope (CD-SEM). However, CD is a complex function of both dose and focus. It is difficult to establish based on CD alone, if CD variation is due to dose or focus having moved from its respective set point. Furthermore, since the lithography process is routinely optimized to result in the least possible CD variation through the process (dose and focus) window, such observable CD variations are very small, rendering conventional lithography process monitoring with CD-SEM both ineffective and inefficient.
Other CD-related process monitoring approaches include, but are not limited to scatterometry techniques (ellipsometry, variable angle, reflection) using complex look-up libraries, and optical CD techniques utilizing optical metrology tools and dual tone arrays to indirectly measure the critical dimensions using line-end shortening techniques.